Biodegradable materials are kinds of new materials capable of being degraded and used by environmental organisms (microorganisms). A new generation of biodegradable materials takes polylactic acid as representative, the polylactic acid is divided into poly-L-lactic acid and poly-D-lactic acid, which are obtained by the polymerization of L-lactic acid monomer or D-lactic acid monomer, respectively. It is estimated that the global demand for polylactic acid will reach 15 million tons per year by 2020, and the polylactic acid will be the most likely substitute of Polyethylene terephthalate (PET) and Polystyrene (PS) which have an annual demand of 50 million tons.
The poly D-lactic acid material polymerized by high-quality D-lactic acid as raw material can be a good alternative to fiber, plastic and other products polymerized by common chemical product. Especially in the field of high-end consumer products, such as in the processing and polymerization of diapers built-in gaskets, cigarette filter heads and other materials, a unique biomass material characteristic, biological compatibility and non-toxic and harmless characteristics of it could greatly improve the quality of related products, and the market space is huge. A new product prepared by 3D printing technology using the poly-D-lactic acid as raw material has environmental protection, good mechanical property, safety and other multiple advantages, so it is widely used in cars, disposable supplies, electronics, medical and other fields. More worth noting is that the blending of the high-quality polylactic acid (PLA) polymerized by the extremely high optical pure D-lactic acid and L-lactic acid with the biodegradable butylene succinate-butylene adipate copolymer (PBSA) can greatly improve the strength and toughness of biodegradable material, and expand the application field of related products. At present, trial production of blending products of PLA and PBSA has been completed by companies such as BASF and the like, a series of new biodegradable materials with excellent performances were successfully launched, and the relevant development greatly increased the market requirements of the extremely high optical pure D-lactic acid and L-lactic acid.
Due to its own characteristics and the complexity of the medium required, the optical purity of lactic acid produced by traditional lactic acid producing strain such as Rhizopus Oryzae and Lactobacillus Acidophilus is unable to meet the requirement of polymer grade and cannot be used for scale production of extremely high optical pure D-lactic acid and L-lactic acid. Recombinant yeast and recombinant Escherichia coli are able to produce extremely high optical pure D-lactic acid and L-lactic acid, and also have the advantages of low nutritional requirements, being easy to be cultured with a high density and popularized in industrial-scale, which made the recombinant yeast and recombinant Escherichia coli became hot research of producing strains of extremely high optical pure D-lactic acid and L-lactic. Compared with the recombinant yeast, multiple genetically modified E. coli is able to produce extremely high optical pure and extremely high chemical pure lactic acid. Due to the culture temperature of the E. coli is obviously higher than that of the yeast, the fermentation period of E. coli for lactic acid production is greatly shortened compared with recombinant yeast. Therefore, the recombinant E. coli is considered to be the most ideal strain for producing extremely high optical pure D-lactic acid and L-lactic acid in industrial scale.
Especially in recent years, E. coli has been widely recognized as a D-lactic acid producing strain. If E. coli can also be used for production of high-optical-purity L-lactic acid, it will be important significance for alternate production of D-lactic acid and L-lactic acid on the same production line. In addition, the rapid development of high-quality lactic acid monomer and the polylactic acid manufacturing industry is also facilitated.
A number of well-studied researches about the production of extremely high optical pure D-lactic acid by recombinant E. coli as a producing strain have been carried out (Zhou L. et al, Current Microbiology, 2011, 62: 981-989; Zhu Y. et al., Applied Environmental Microbiology, 2007, 73: 456-464; Zhou S. et al., Applied Environmental Microbiology, 2003, 69: 399-407; Zhu J. et al., Applied Microbiology and Biotechnology, 2004, 64: 367-375, Zhu J. et al., Metabolic Engineering, 2005, 7:104-115; Bunch P. K. et al., Microbiology, 1997, 143: 187-195). For example, 1) Strict anaerobic fermentation to improve acid production efficiency (Li et al., Applied Microbiology and Biotechnology, 2002, 60: 101-106); 2) Low-stirring limited oxygen fermentation under stopping ventilation condition to improve acid production efficiency (Zhou L. et al., Current Microbiology, 2011, 62:981-989); 3) Maintaining micro-aerobic fermentation conditions to increase acid production efficiency (Tian K et al., 2012 African Journal of Biotechnology, 11(21): 4860-4867; Zhou L et al., Biotechnology letters 2012, 34:1123-1130); 4) Adopting suitable culture temperature for cell growth and sub-suitable growth temperature limiting the cell growth to increase fermentation and acid production efficiency (Niu D et al., Microbial Cell Factories 2014, 13:78-88; Zhou L et al., Metabolic Engineering, 2012, 14:560-568); 5) Using different carbon sources in the cell growth and fermentation of acid to improve the acid production efficiency and other method (Zhu L et al., Applied Environmental Microbiology, 2007, 73: 456-464).
The inventor's patent issued earlier (Chinese Patent, Patent Number: ZL201210102731.8) disclosed that introducing a temperature regulation element in the gene transcription level, and the synthesis efficiency of the D-lactic acid is greatly improved by matching with the fermentation temperature regulation and control strategy. Based on this, the present disclosure introduced a regulatory mechanism of cell growth quantity control, and a double-switch mechanism of a cell growth process and a D-lactic acid synthesis process is formed, resulting in further improvement of the D-lactic acid synthesis efficiency.